There are many situations in industrial and analytical chemistry where a flowing stream of chemical fluid needs to be controlled, in a manner in which the risk of contamination of the flowing stream by contact with surfaces must be minimized. Examples include automatic flow monitoring systems and meters, analytical apparatus, gas chromatography, medical gas supply lines and the like. In such cases, it is necessary to provide flow control valves in which all the surfaces which contact the flowing chemical fluid, including the valve operating member, are constructed or at least have surfaces covered with inert metal. In many of these cases also, particularly in the case of flow control valves for use with analytical apparatus, very small size valves of high precision are required, to handle analysis of flowing fluids with precision capable of detection and analysis of fluid contaminants of the order of a few parts per billion.
Specific examples of such applications are high performance liquid chromatography (HPLC) and gas chromatography. In both of these applications, small scale, high precision fluid flow control valves are needed. Especially in gas chromatography, where small amounts of contaminants are required to be detected and analyzed, it is necessary to have a flow control valve which presents chemically inert surfaces to the gas flow.
Gas chromatography is a technique by which a mixture of chemical compounds which are in the gaseous or vaporous state are separated from each other by passing them through a column in which they come into contact with an absorptive medium. A stream of gas (known as the carrier gas) passes through the column to provide transport for the chemical compound through the column. Each chemical species has a different level of affinity to be absorbed and will pass through the column in a time which is in proportion to that affinity. In this manner, compounds emerge from the column separated in time.
In modern gas chromatography, it is often necessary to control more than one stream of carrier gas at the same time and to carry out precisely timed switching of streams of gas. To accomplish this, a number of highly specialized valves are needed. It is important that the valves be made of a chemically inert material for which the chemical compounds under analysis have very low affinity and to which they do not readily diffuse. It is also important that internal volume within a gas chromatography system should be kept to a minimum. This in turn means that the fluid flow control valves must have low internal volume (dead volume), in order to provide the necessary degree of sensitivity.